1. Field of the Invention
The present invention relates to a water treatment system and process and, more particularly, to a water treatment system and process for producing purified water for high purity applications.
2. Description of the Related Art
High purity water is required in many industries such as the chemical industry, the foodstuffs industry, the electronics industry, the power industry, and the pharmaceutical industry. Typically these applications require treatment of a source water supply (such as from a municipal water supply) to reduce the level of contaminants. These treatment techniques can include distillation, filtration, ion exchange, reverse osmosis, photooxidation, ozonation, and combinations thereof.
In an effort to decrease biological contaminants in municipal water supplies, especially in warmer environments, the addition of chloramine to municipal water supplies has become commonplace. While effective as a biocidal agent, chloramine becomes a contaminant itself in certain industrial applications requiring high purity water. Furthermore, chloramine is difficult to remove from water without producing other unwanted contaminants such as ammonia. Municipal water supplies can also contain unacceptable levels of dissolved carbon dioxide, boric acid, silicic acid (hydrated silica) and/or organic materials. These weakly ionized and organic materials are also difficult to remove from water.
Nuclear and fossil-fuel power plants, in particular, have stringent water quality requirements to reduce corrosion and scaling and the associated expensive downtimes. In pressurized water reactor nuclear plants, for example, high-purity water is important in reducing corrosion in steam generators. In boiling water reactor nuclear plants, high-purity water is important in maintaining water quality in the nuclear reactor. Traditionally, makeup water treatment systems for power plants have relied almost exclusively on various combinations of filtration, ion-exchange, and reverse osmosis.
Similarly, the pharmaceutical industry requires various degrees of purified water for use in drug manufacture, injection of drugs, irrigation, and inhalation. The United States Pharmacopoeia (USP) lists standards for the various types of water used in the pharmaceutical industry, including purified water, sterile purified water, water for injection, sterile water for injection, sterile bacteriostatic water for injection, sterile water for irrigation, and sterile water for inhalation.
While drinking water is not covered by the USP, water that complies with the U.S. Environmental Protection Agency (EPA) drinking water regulations is the prescribed source water for the production of pharmaceutical grade waters. Purified water can be used to process certain drugs, particularly as a cleaning agent for equipment and in the preparation of certain bulk pharmaceuticals. Purified water, according to the USP, must meet almost all of the same purity requirements as water for injection with the exception of bacteria and pyrogen levels. As noted, purified water is produced using drinking water as the source water, which is purified using pretreatment equipment followed by at least one of ion exchange, reverse osmosis, and distillation.
Sterile purified water is not used in any drug that will be introduced directly into the bloodstream. Purified water is made sterile by heating it to a minimum temperature of 121xc2x0 C. for at least 15 minutes. Water for injection, according to the USP, may be used in the production of certain drugs, also as a rinsing agent for certain equipment and the preparation of certain bulk pharmaceuticals. Water for injection can be introduced directly into a patient""s bloodstream and, therefore, must meet all purified water standards and additionally meet endotoxin limits. In the processing of water for injection, a reverse osmosis or distillation unit must be used. Relatedly, sterile water for injection is packaged in volumes not larger than 1 liter and is made sterile as noted above.
Typically, sterile water for injection is used to dilute drugs which will be introduced into the bloodstream. Sterile bacteriostatic water for injection is similar to sterile water for injection that is packaged in volumes not larger than 30 milliliters and to which is added antimicrobial agents. Sterile water for irrigation is used during surgical procedures to flush tissue within the body. Lastly, sterile water for inhalation is similar to sterile water for injection that is used in inhalers and in the preparation of inhalation solutions.
FIG. 1 illustrates a schematic process flow diagram of a prior art water treatment system 10 for producing purified water and/or water for injection for use in pharmaceutical applications. As shown, feedwater, typically municipal drinking water, is fed through line 12 to a media filter unit 14 to remove bulk particulate material. The water is then passed through a water softener 16, most typically an ion exchange unit. The ion exchange unit is typically a sodium-cycle cation exchange unit that is used to remove scale-forming cations such as calcium and magnesium. In addition, water softener 16 serves to remove double and triple charged cations and reduce the tendency for coagulation of colloids that could foul downstream reverse osmosis membranes. The water is then passed through a heat exchanger 18, which is typically used in system 10 if the source water is from a surface water source such as a lake or river.
The water is passed from heat exchanger 18, if used, to a dechlorination unit 20 that includes an activated carbon bed to remove chlorine, which is typically present in the municipal drinking water that serves as the source water for the system. A dechlorination unit is necessary because the most commonly used reverse osmosis membranes, thin film composite polyamide membranes, typically have low tolerance to oxidizing agents such as chlorine. The water is then passed to a cartridge filtration unit 22, which provides a final filtration to protect the reverse osmosis membranes from fouling or other damage caused by relatively large particles generated from upstream equipment. The water is then passed to a reverse osmosis unit 24, which typically removes greater than 98 percent of dissolved substances from the feedwater. Although not shown, a double-pass configuration of reverse osmosis units can be used to achieve high quality purified water. The permeate from the reverse osmosis unit(s) is then passed to a distillation unit 26 for the production of water for injection. A storage tank 28 may also be provided to store the distilled water prior to its use in production and/or packaging in unit 30.
As noted above, municipal drinking water is either chlorinated or chloraminated to control pathogenic microorganisms. In recent years many municipalities have changed methods of water supply disinfection from chlorine to chloramines to reduce the formation of trihalomethanes (THM""s), which the EPA currently limits to 100 parts per billion in potable water. THM""s are formed by the reaction of chlorine with organic substances. Chloramines are formed by the addition of chlorine, which forms hypochlorous acid, and ammonia to the water. Chloramine form (mono, di, tri) is a function of pH and the chlorine/ammonia ratio, with monochloramine predominating at pH 7 or higher; dichloramine predominating at pH 4 to 5.5; and trichloramine predominating at pH 3 to 4.5 by the following reactions:
Cl2+H2Oxe2x86x92HCl+HOCl (hypochorous acid)
NH3+HOCl⇄H2O+NH2Cl (monochloramine)
NH2Cl+HOCl⇄H2O+NHCl2 (dichloramine)
NH2Cl+HOCl⇄H2O+NCl3 (trichloramine)
A dechlorination unit, including an activated carbon bed, is effective for the reduction of free aqueous chlorine by the following reactions:
C+HOClxe2x86x92CO+H++Clxe2x88x92and
C+OClxe2x88x92xe2x86x92CO+Clxe2x88x92
and has also been used to remove chloramines, with considerably longer contact time, as follows:
xe2x80x83NH2Cl+H2O+C.xe2x86x92NH3+Clxe2x88x92+H++CO, and
2NH2Cl+CO.xe2x86x92N2(g)+H2O+2Clxe2x88x92+2H++C.
The production of both ammonia and nitrogen gas, however, indicates that the activated carbon bed is not sufficient by itself to achieve a low concentration of both chloramine and ammonia. Moreover, ammonia is not effectively removed from water by reverse osmosis treatment.
As government regulations become more stringent and the demand for high purity water continues to grow, new and improved methods of treating municipal water to obtain high purity water are needed. To date, there is no effective method of removing chloramines, as well as weakly ionized species and organic materials, in the same system.
Accordingly, the present invention is directed to a water treatment system and process for providing high purity water. Advantages of the present invention include its ability to substantially reduce or eliminate the presence of weakly ionized and/or organic materials, including their equilibrium constituents, from water; its ability to effectively remove chloramine from water; and its ability to substantially remove the aforementioned materials while providing high resistivity water.
The system includes an inlet for introducing a feedstream into the system. The feedstream is treated downstream of the inlet in a first chemical treating unit to maintain the pH of the feedstream below about 7 to substantially reduce a concentration of weakly basic components in the water by chemical conversion to a more ionized state. A first water treatment unit is positioned downstream of the first chemical treating unit to substantially remove ionized components in the stream, and produce a first product stream. The first product stream is treated downstream of the first water treatment unit in a second chemical treating unit to maintain the pH of the first product stream above about 7 to substantially reduce a concentration of weakly acidic components in the water by chemical conversion to a more ionized state. A second water treatment unit is positioned downstream of the second chemical treating unit to substantially remove the ionized components, and produce a second product stream. The high purity water is removed from the system through a purified water outlet positioned downstream of the second water treatment unit.
In various embodiments, the first water treatment unit can be selected from the group consisting of a media filter unit, a water softener unit, a dechlorination unit, and combinations thereof. The second water treatment unit can be a single reverse osmosis unit, or staged reverse osmosis units wherein a second reverse osmosis unit is positioned downstream of the first reverse osmosis unit and upstream of the purified water outlet. An electrodeionization unit may be used in the water treatment system, in place of or in series with the reverse osmosis unit(s). The system can further include a media filter unit, a water softener unit, and/or a dechlorination unit positioned upstream of the first chemical treating unit. A mixed bed polisher unit can be positioned downstream of the purified water outlet.
In one embodiment of the water treatment system of the present invention, the system includes an inlet for introducing a feedstream into the system. The feedstream is treated downstream of the inlet in a chemical treating unit to maintain the pH of the feedstream below about 7 to substantially reduce a concentration of weakly basic components in the water by chemical conversion to a more ionized state. A first reverse osmosis unit is positioned downstream of the chemical treating unit to substantially remove the ionized components in the stream, and produce a first product stream. An electrodeionization unit is positioned downstream of the first reverse osmosis unit to produce a second product stream. A second reverse osmosis unit is positioned downstream of the electrodeionization unit to produce a third product stream. The high purity water is removed from the system from a purified water outlet positioned downstream of the second reverse osmosis unit.
In another embodiment of the present invention, a feedstream enters the system through an inlet and is treated in a first reverse osmosis unit to produce a first product stream. A first water softener unit positioned downstream of the first reverse osmosis unit receives the first product stream and produces a second product stream. The second product stream is treated in a first chemical treating unit positioned downstream of the first water softener unit to maintain the pH of the stream below about 7 to substantially reduce a concentration weakly basic components by chemical conversion to a more ionized state. The pH adjusted second product stream passes through a second reverse osmosis unit to substantially remove ionized components and produce a third product stream, which exits the system through a purified water outlet. Optionally, the system can include a second chemical treating unit positioned upstream of the first reverse osmosis unit to maintain the pH of the feedstream above about 7 to substantially reduce a concentration of weakly acidic components by chemical conversion to a more ionized state.
In another embodiment, the system includes a water inlet for introducing a feedstream into the system. The feedstream passes sequentially through a media filter unit, a first water softener unit, and a dechlorination unit, before passing through a first chemical treating unit wherein the pH of the feedstream is maintained below about 7 to substantially reduce a concentration of weakly basic components by chemical conversion to a more ionized state. A second water softener unit is positioned downstream of the first chemical treating unit to substantially remove ionized components and produce a first product stream. A second chemical treating unit is positioned downstream of the second water softener unit to maintain the pH of the first product stream above about 7 to substantially reduce a concentration of weakly acidic components by chemical conversion to a more ionized state before the stream enters a first reverse osmosis unit positioned downstream of the second chemical treating unit to substantially remove ionized components and produce a second product stream. The second product stream passes through a second reverse osmosis unit positioned downstream of the first reverse osmosis unit before exiting the system through a purified water outlet.
In another embodiment the water treatment system includes a water inlet for introducing a feedstream into the system and a first chemical treating unit to maintain the pH of the feedstream below about 7 to substantially reduce a concentration of weakly basic components by chemical conversion to a more ionized state. A water softener unit is positioned downstream of the first chemical treating unit to substantially remove ionized components and produce a product stream. A purified water outlet, positioned downstream of the water softener unit, allows the product stream to exit the system.
In another embodiment of the present invention, a water treatment process is provided. The process includes the steps of a providing a feedstream to be treated and maintaining pH of the feedstream below about 7 to substantially reduce a concentration of weakly basic components by chemical conversion to a more ionized state. The feedstream is then treated in a first water treatment unit to substantially remove ionized components and produce a first product stream. The first water treatment unit can be selected from the group consisting of a media filter unit, a water softener unit, a dechlorination unit, and combinations thereof. The pH of the first product stream is then maintained above about 7 to substantially reduce a concentration of weakly acidic components by chemical conversion to a more ionized state before the stream is treated in a second water treatment unit to substantially remove the ionized components and produce a second product stream. The second product stream is then removed from the second water treatment unit.
In another embodiment of the present invention, the water treatment process includes the step of maintaining the pH of a feedstream below about 7 to substantially reduce a concentration of weakly basic components by chemical conversion to a more ionized state. The feedstream then enters a first reverse osmosis unit to substantially remove ionized components and produce a first product stream. The first product stream is then introduced into an electrodeionization unit to produce a second product stream. The second product stream is treated in a second reverse osmosis unit to produce a third product stream.
In another embodiment, the process includes treating a feedstream in a first reverse osmosis unit to produce a first product stream, before treating the first product stream in a water softener unit to produce a second product stream. The pH of the second product stream is maintained below about 7 to substantially reduce a concentration of weakly basic components by s chemical conversion to a more ionized state before the stream enters a second reverse osmosis unit, which substantially removes ionized components and produces a third product stream. Optionally, the pH of the feed stream is maintained above about 7 to substantially reduce a concentration of weakly acidic components by chemical conversion to a more ionized state before treating the stream in the first reverse osmosis unit. To improve output quality, product recovery through the second reverse osmosis unit can be maintained at less than about 90%, flux across the reverse osmosis membranes can be increased, and/or the temperature of the process stream can be maintained below about 20xc2x0 C.
In another embodiment, the water treatment process includes the steps of providing a feedstream to be purified, introducing the feedstream into a chemical treating unit to maintain the pH of the feedstream below about 7, and treating the feedstream in an activated carbon filter located immediately downstream of the chemical treating unit to produce a product stream. The product stream is then removed from the activated carbon filter.